In the commercial production of ammonia synteesis gas the common practice is to subject natural gas or other gaseous hydrocarbon mixture) to two stage reforming, employing steam as a reactant in the first or primary stage and air or oxygen in the secondary stage. In accordance with one of the known methods, the primary reforming operation is carried out under conditions such that the primary steam reformate contains a mnor quantity of unconverted methane usually in the order of around 10%) which is reacted in the secondary reformer using an amount of air to furnish nitrogen in about stoichiometric proportion for production of NH.sub.3. In another known process the primary reforming operation is carried out under conditions such that a much higher amount of unconverted methane is left in the primary reformate (in order of about 16 to 22%) which contained methane is converted in the secondary reformer using significantly large amounts of air, which amounts generally exceed stoichiometric nitrogen requirements for NH.sub.3 production.
In each of these known commercial processes the raw ammonia syngas obtained is further processed by water gas shift reaction for conversion of carbon monoxide to the dioxide, followed by removal of carbon dioxide thus formed by extraction with monoethanolamine (MEA). alkaii carbonate or other physical absorbent solution. Residual carbon oxides. which are poisons to the ammonia synthesis catalysts are converted to methane via conventional methanation.
The synthesis gas thus obtained, which is introduced into the ammonia synthesis loop, is relatively free of CO and CO.sub.2 but contains small amounts of impurities such as methane and argon. While the contained impurities are inerts in the ammonia synthesis process, they must be purged to eliminate their buildup in the synthesis loop.
In the above first described process a large primary reformer is needed and adequate provision for convection waste heat recovery. In the second described process a larger secondary reformer is required plus a larger size air compressor, in addition to means for purging surplus nitrogen from the ammonia synthesis recycle loop to obtain the desired H.sub.2 :N.sub.2 stoichiometric ratio. Thus. each of the current processes for production of ammonia syngas has its drawbacks. In addition to the capital and operating costs incurred as a result of increased size of the primary and/or secondary reformers and for convection waste heat recovery. there are also increased energy losses through the walls and stack of the reformers. The extent of conversion of syngas to ammonia depends upon the partial pressure of the reactants nitrogen and hydrogen. The required purging of accumulating inerts is accompanied by a loss of valuable reactants. Also. because of the buildup of inerts a larger recycle stream must be used, which necessitates greater recompression and a larger sized synthesis reactor and loop.
In addition to the foregoing drawbacks, the absorption method presently employed in the removal of CO.sub.2 from the reformate is energy intensive and adds significantly to the costs of ammonia production.
Various approaches have been proposed for overcoming the drawback above described. One such proposed approach is to treat the purge stream from the ammonia synthesis loop to recover hydrogen therefrom, which can be recycled to the synthesis gas being charged to the ammonia conversion operation. This approach solves only that part of the problem concerning reactant loss in the purge gas. The known separation and recovery of the hydrogen from the recycle may be effected by use of semi-permeable membrane, cryogenic fractional distillation or selective pressure swing adsorption (PSA).
A second approach proposed and offered commercially. is based upon separate production of pure hydrogen, to which pure nitrogen is subsequently added to provide the required 3:1 H.sub.2 /N.sub.2 ratio.